I rebuilt my 250r motor this week. The bottom end was rebuilt last year, I had the cyinder honed and got new piston kit, gaskets and o-rings. Cleaned and rebuilt the powervalve. Put it together, turned over fine, today I put the new powervalve parts on, still spun over fine. Put the motor in my trike, oil, coolant, gas tank, plastic, leaving the only thing left to do in gas it up. Go to kick it and the kicker moves about 5" and its locked. Tore it down, the piston was hard to get out, metal shavings in the crank case. Look a the cylinder in better light and it also needs to be re-plated. Where the piston stuck, it gouged the cylinder wall. Great news. Don't understand, how, why inside such a short time did this happen. I've rebuilt many motors, this isn't my first. I didn't take apart the clutch side yet, when I put a new clutch, springs and bolts in, it was mint.